A vehicle is equipped with a water temperature sensor for detecting the cooling water temperature of an engine. The cooling water temperature information detected by the water temperature sensor is used as input information for various controls for controlling the operation condition of the engine such as an idle speed control (hereinafter referred to as ISC) for controlling an idling speed of the engine always to an optimal value, a fuel injection control (hereinafter referred to as fuel control) for determining the amount of fuel according to the operation condition of the engine to control an air/fuel ration to an optimal value, and an ignition timing control (hereinafter referred to as ignition control) for determining an optimal ignition timing.
An example of the water temperature sensor will be described with reference to Fig. 12. The water temperature sensor comprises a thermistor 1 which is a negative resistance element for varying the voltage dividing ratio and two voltage dividing resistors 2 (2.2 k.OMEGA. and 39 k.OMEGA. in the figure) connected in series. A 5 V reference voltage is divided to 4.7 to 0.1 V according to a change in 46 water temperature, that is, a change in resistance of the thermistor 1.
The divided voltage according to the cooling water temperature generated by the water temperature sensor is supplied as input information for various controls to an electronic control unit (hereinafter referred to as ECU) through an A/D converter (not shown).
However, in such a water temperature sensor, the sensor output may become out of the setting range of 4.7 to 0.1 V. That is, even with a normal cooling water temperature, an open circuit may occur in the thermistor 1 to increase the resistance to nearly an infinite value, or a short-circuit may occur to decrease the resistance of the thermistor 1 nearly to zero. Such phenomena may lead to outputs of information of a much higher or lower temperature than the actual temperature, resulting in a malfunction in various controls for controlling the operation condition of the engine.
For example, if the resistance of the thermistor 1 is nearly zero due to a short-circuit, a too lower water temperature information will be outputted, which results in the following malfunctions:
(1) ISC
An intake air control valve for controlling the intake air amount of the engine moves so that in intake path is opened to increase the engine speed and the cooling water temperature. As a result, the engine speed is increased exceeding the desired value.
(2) Fuel control
The too lower water temperature information promotes unnecessary enrichment of the fuel/air mixture, decreasing the mileage and resulting in increased contamination of exhaust gas.
(3) Ignition control
The condition is determined as a cold-state operation, and the ignition timing is advanced, which may lead to occurrence of a knocking.
To eliminate such a problem, when a water temperature sensor output voltage of higher than 4.7 V is detected which indicates an open circuit or when a voltage of lower than 0.1 V is detected which indicates a short-circuit, it is judged as a failure in the water temperature sensor, various controls are performed by a dummy signal output corresponding to a cooling water temperature of 80.degree. C., and a diagnosis check lamp is lit to inform the malfunction to the driver.
Other than the above-described open circuit and short circuit, there is a case that the water temperature sensor is momentarily cut off. This phenomenon is that, as the output of the water temperature sensor decreases with increasing water temperature as shown in FIG. 13 (a), halfway the water temperature sensor output momentarily increases to give an impulse output. This momentary cut-off is caused by a momentary disconnection due to vibrations of the like of any of a plurality of connectors 3 from the water temperature sensor to the ECU. To reduce effects of such a momentary cut-off, the water temperature sensor output is averaged. Specifically, where WT.sub.n is water temperature information used for a n'th control, WT.sub.R is water temperature information of a n'th the detection, and K is a constant, water temperature is obtained by the following equation: EQU WT.sub.n =(1-K) WT.sub.n-1 +K.multidot.WTR
In other words, averaging is made so as to eliminate an impulse output shown in FIG. 13 (a).
Although open circuit, short-circuit, and momentary cut-off can be treated by the above detection methods and averaging, there are more important problems. Open circuit and shor-circuit are extreme cases, and momentary cut-off can be removed by averaging. However, there is a case of a gradual change with time, which tends to be looked over and result in a major malfunction when it becomes conspicuous. For example, there is a phenomenon in which a resistance is added to the thermistor as shown in FIG. 14(a). As shown in FIG. 14(a), when a poor soldering or a poor contact occurs in the water temperature sensor and its associated circuit which contains soldered portions and conncetors 3 (FIG. 13(a)) casued by expansion or oxidation due to an increase in temperature or vibrations, it becomes equivalent to the addition of resistances R.sub.1 to R.sub.4 though it does not result in a momentary cut-off or open circuit, and the resistance of the thermistor 1 tends to always become increased. As a result of this increase in resistance, from the characteristics shown in FIG. 14(b), despite that the water temperature sensor output must be decreased with increasing cooling water temperature, the water temperature sensor output remains high even when the cooling water temperature increases.
As a result, despite that the actual cooling water temperature is 80.degree. C., for example, and the engine is warmed up, if the high output voltage is judged as that from normal characteristics, the cooling water temperature is determined as to be, for example, 20.degree. C., resulting in the above-described malfunctions of various control systems for a low cooling water temperature.
Therefore, in performing various controls in response to the cooling water temperature measured by a water temperature sensor using a thermistor, an improvement has been required for malfunctions in various control systems not only when an open circuit or short-circuit occurs but also when the resistance abnormally increases.